Words that rhyme have the same sound. For example, 'Cheese' and 'peas' are words that both have the same sound. You can write rhyming poems by using pairs or groups of words that use the same sounds.
Although, I think he should probably pick a different branch! The words are all spelt with the letters O U G H, but the sound is different. That monkey is saying 'ough ough' like you would in ...